1. Field of the Invention
The present invention relates to a multi channel optical transmitter/receiver module and, more particularly, to a multi channel optical transmitter/receiver module having precise alignment of optical devices and optical fibers.
2. Description of Related Art
Recently, communication systems designers are vigorously adapting their designs for the use of optical fiber technology in various communication fields. Optical communication systems enable use of high frequency signals and suffer less signal loss than conductor based technologies and are therefore better suited for the high bandwidth communications that are increasingly in demand. Optical communication systems are suitable to use in high speed-long distance transmission systems.
During optical transmission of data, one channel of series data is generally utilized for transmitting parallel data on N channels. In this case, the transmission speed of the series data should be at least N times faster than each of the parallel data channels. High speed transmission circuits require expensive equipment; therefore, multiple transmission channels are often utilized to reduce the burden of a high speed transmitting circuit. In order to use multiple optical channels, a plurality of optical transmission systems, each including a light source, an optical fiber, and light detector, are required. For multi channel optical transmitter/receiver modules, an accurate alignment of optical fibers with sources and detector is required not only for each channel but also for adjacent channels. Therefore, multi channel optical transmitter/receiver modules need an optical connector which is highly accurate and, consequently, is more complicated than that of a single channel optical transmitter/receiver module.
FIG. 3 is an exemplary schematic diagram illustrating an active alignment method for a multi channel optical transmitter/receiver module 101. In order to arrange laser diodes 100, for example, with respect to optical fibers 110, laser diodes 100 are first fixed so that they are separated by regular, usually uniform, intervals. Next, optic fibers 110 are fixed on a block 120 having grooves with the same regular intervals with which the laser diodes have been fixed. Then, laser diodes 100 and optical fibers 110 are aligned by moving block 120 with respect to laser diodes 100. Block 120 can be moveable in all three directions. An optimal alignment between optical fibers 110 and laser diodes 100 can be achieved by monitoring the optical output power from each optical fiber of optical fibers 110 while moving block 120. When the output power from each of optical fibers 110 is maximized, block 120 can be fixed relative to diodes 100. This method is referred to as the active alignment method because the maximum output power is sought by monitoring the optical output power from fibers 110. The active alignment method can approach the optimum alignment, however it requires expensive equipment and a lot of labor hours to accomplish. Further, the active alignment method does not lend itself to systems where plugable connectors are desirable.
FIG. 4 is an exemplary schematic diagram illustrating a passive alignment method for a multi channel optical connector module 201. In contrast to the active alignment method illustrated in FIG. 3, the passive alignment method adjusts the locations of optical fibers 221 without monitoring any optical output power. Multi channel optical connector module 201, utilizing the passive alignment method, includes an optical device array block 210 with optical devices 200, each electrically coupled to one of electrical conductors 211, arranged to have regular, uniform, intervals and a multi channel optical fiber block 220 having optical fibers arranged with the same regular intervals as that of optical devices 200 of optical device array block 210. Optical device array block 210 can be fixed on a substrate (not shown) by soldering. Multi channel optical fiber block 220 can be plugable. Optical fibers 221 is then aligned with optical devices 200 when multi channel optical fiber block 220 is plugged into optical device array block. Optical devices 200 can be laser diodes or photodiodes. Even though the passive alignment method is not optimized as with the active alignment method, it has the advantage of being faster (requiring fewer labor hours), requires less expensive equipment, and therefore is less expensive to perform.
FIGS. 6 and 7 illustrate a conventional method of producing connector 201 of FIG. 4. Typically, an optical transmitter/receiver module will include two connectors such as connector 201 of FIG. 4 arranged such that light sources in one connector are coupled with light detectors in the other connector via optical fibers.
FIG. 6 shows an assembly diagram of a conventional method of performing the passive alignment method for multi channel optical fiber block 220. A connector block 310 is grooved with grooves 311 having uniform intervals, and optical fibers 320 are inserted in the grooves. Optical fibers 320 are fixed in place by a cover 300, which can also be grooved with grooves 312 having the same uniform intervals as connector block 310. Connector block 310 is usually made from a plastic material for ease of manufacturing and lowered cost. End facets 321 of optical fibers 320 are usually smoothly polished in order to facilitate the coupling of light into and out of optical fibers 320.
As discussed so far, the conventional multi channel optical fiber block is generally made of a plastic molding forming a solid body with grooves for laying optical fibers. The plastic molding has advantages for mass production and is inexpensive to produce, but results in large alignment errors in placement and spacing of optical fibers 320. Because the alignment error of the plastic is large, a 0.5 mm or larger diameter plastic optical fiber should be used, enabling light to be easily coupled into the optical fiber from laser diodes. If an optical fiber having larger diameter than the diameter of the light receiving aperture of the photo diode is used, however, all of the light coming from the laser diode could be entered to the optical fiber, but all of the light transmitted out of the optical fiber would not be entered to the photodiode. Consequently, the overall loss of light through the system is increased.
FIGS. 7(a) through 7(c) show an assembly diagram of a conventional method of performing the passive alignment method for an optical device array block 210. As with multi channel optical fiber block 220 of FIG. 6, optical device array block 210 can be made from molded plastic. As shown in FIG. 7 (a), a thin metal plate 420 for laying optical devices 430 and a set of metal leads 410 for transmitting electric signals is insterted and fixed into an array block 400. Next, as shown in FIG. 7(b), optical devices 430, which can be laser diodes or photodiodes, are affixed on the thin metal plate 420 by using a conductive adhesive such as, for example, a silver epoxy. Optical devices 430 and a bundle of the optic fibers 320 as shown in FIG. 6 are each arranged with the same uniform intervals. Electrodes are formed at the top and bottom surfaces of optical devices 430. Because all of the bottom surfaces of optical devices 430 are affixed on metal plate 420 by the conductive adhesive, the bottom surfaces of the optical devices 430 form a common electrode.
As shown in FIG. 7 (c), the top surface of each optical device 430 is connected to a corresponding one of metal leads 410 by a wire 431, usually a gold wire. The bottom surface of each of optical devices 430 is affixed to metal plate 420, which forms a common electrode and is also connected to one of metal leads 410 by a wire 431, usually a gold wire. Generally, gold wire is affixed to optical device 430 and metal plate 420 by ball bonding using ultra sonic techniques, and gold wire is affixed to metal lead 410 with silver epoxy.
TABLE 1 shows the result of a calculation for an allowable tolerance of the alignment depending on the various diameters of optical fibers and a coupling efficiency between the optical fiber and optical devices. The allowable tolerance for alignment between a laser diode and an optical fiber is based on the requirement that more than about 90% of the maximum optical output of the laser diode be coupled into the optical fiber. The allowable tolerance of alignment between an optical fiber and a photo diode is based on the requirement that more than about 90% of the maximum light output from the optical fiber be coupled into the photo diode.
In the calculations of Table 1, the divergence angle of the laser diode beam is assumed to be about 15°. The diameter of the light receiving aperture of the photodiode is assumed to be about 200 μm. Additionally, the laser diode is separated by about 450 μm from the optical fiber.
TABLE 1Laser diode-Optical fiber-Laser diode-Optical fiber-Optical fiberPhoto diodeOptical fiberPhoto diodeTotalAllowableAllowableMaximumMaximumMaximumOptical fibertolerance oftolerance ofcouplingcouplingCouplingcore diameteralignmentalignmentefficiencyefficiencyefficiency0.5mm±140 μm±90 μm100% 21%21%0.25mm ±40 μm±45 μm 90% 67%60%0.0625mm ±20 μm±65 μm 16%100%16%
If a 0.5 mm core diameter plastic optical fiber is used, it would be possible to manufacture the connector having approximately 100 μm of allowable tolerance of alignment between the optical fiber and the laser diode by plastic molding. However, only 21% of the light output from the optical fiber can be coupled into the photodiode. Alternatively, if a 0.25 mm core diameter plastic optical fiber is used, 67% of the light output from the optical fiber can be coupled to the photodiode. The decreased diameter of the optical fiber can bring three times the signal to the photo diode without increasing the output of the laser diode; however, the allowable tolerance of alignment between the optical fiber and the laser diode would be reduced by a factor of about 0.3 that of the 0.5 mm diameter plastic optical fiber. It is very difficult to manufacture such a connector and satisfy the allowable tolerances with plastic molding. The passive alignment method is generally accomplished with plastic optical fiber having relatively large diameters, generally about 0.5˜1.0 mm, for properly transmitting the optical signal.
Moreover, if a 0.0625 mm diameter multi mode silica optical fiber is used, it is extremely difficult to satisfactorily manufacture the connector with the required reduced alignment tolerances by plastic molding. However, even though the amount of the output of the laser diode actually coupled into the multi mode silica optical fiber is small, all of the light coming out from the optical fiber can be coupled into the photodiode. Thus, the maximum output of the photodiode is almost same as that of the 0.5 mm diameter optical fiber. Moreover, the silica optical fiber is essential for high speed-long distance signal transmission because silica optical fiber has almost no loss of power and a high cut-off frequency compared with plastic optical fiber.
The multi channel optical connector module manufactured by the conventional passive alignment method shown in FIGS. 6 and 7 has the advantage of being faster and simpler to perform than the active alignment method. However, there are some problems in the conventional passive alignment method.
For the multi channel optical fiber array block, the alignment error between the optical devices 430 (FIG. 7) and the optical fiber 320 (FIG. 6) is increased because connector block 310 and grooves 311 for laying the optical fibers are manufactured as a solid body by plastic molding. Plastic optical fiber can be utilized for reducing alignment error, but plastic optical fiber results in greater signal loss and is not adequate for high frequency transmission.
Additionally, it is very hard to affix optical devices 430 accurately on metal plate surface 420 because optical device array block 400 has no alignment key for arranging optical devices 420 accurately on rectangular metal plate surface 420. The width and height of each of optical devices 430 is generally about 0.3˜0.5 mm. Due to the small size of each of optical devices 430, it is very difficult to affix each of optical devices 430 by the naked eyes. Optical devices 430 are usually affixed with uniform intervals on metal plate 420 using a magnified image through a microscope. Because there are no alignment keys for locating optical devices 430, the accuracy of placement is low. A series of grooves to be used as alignment keys for placement of optical devices 430 could be marked on metal plate 420, but it is not easy to mark grooves that small on metal plate 420, which has a width of approximately 0.5 mm and a length approximately 6 mm. In order to mark a groove having a tolerance of 0.05 mm or below requires extremely precise skills. For measuring a precise distance, an expensive optical vision system would be needed. Besides, the alignment of each of optical devices 430 is required. Therefore, the assembly process will be very complicated and slow.
Additionally, it is not easy to solder a conventional optical device array block 440. Metal plate 420 must be heated to about 200˜250° C. for soldering. But, the optical device array block 440, being made of plastic, has poor heat conductivity. Therefore, heat would not be properly transferred to metal plate 420, which is disposed inside of optical device array block 440. Additionally, heat would deform the plastic material of the optical device array block 440. Additionally, when optical devices 430, which are laid on metal plate 420, are heated at the same time, the heat would melt the solder of the pre-soldered optical devices and cause displacement of the optical devices from the original position. However, soldering is needed because a conductive adhesive used for coupling between optical devices 430 and metal plate 420 causes increased resistance and weakens the transmission of high frequency signals. Soldering the optical devices one by one would be very a painful job. A one-time soldering method is desired to efficiently affix and couple optical devices 430 to metal plate 420. In the one time soldering method, the plurality of arranged optical devices must be fixed during the soldering to avoid displacing the arranged optical devices. However, it is not easy to hold the plurality of arranged optical devices 430 during the soldering process.
It is also very difficult to use a ball bonding method with a narrow surface, such as metal leads 410, when a wire 431 is connected to the metal lead for the conventional optical devices array 440. Thus, soldering is replaced by a conductive adhesive for connection of wires 431. The conductive adhesive weakens the transmission of high frequency signals. Moreover, wide metal plate 420, on which optical devices 430 is fixed, acts as an antenna and both radiates and receives electromagnetic fields, creating a high level of electromagnetic noise.
Therefore, there is needed a multi channel optical connector module capable of being precisely aligned in a fast, cost sensitive fashion. Additionally, there is a need for a multi channel optical connector module having efficient throughput without creating large electromagnetic noise.